1. Field of the Invention
The present invention relates generally to diazo developing apparatus and more particularly to a diazo developing apparatus using a developing gas, e.g. ammonia gas, in a closed system.
2. Description of the Prior Art
Photographic emulsions and liquid photoresists were essential to the early development of transistors and other semiconductor devices. Today the entire semiconductor industry is dependent on the use of photographic emulsions or photoresists for the manufacture of their devices and circuits. Numerous photoetching steps are applied sequentially in order to form the various active and passive components of integrated circuits.
There are many diverse techniques and processes using photoresist in semiconductor fabrication. However, there are basically only two types of resist; negative-acting resists and positive-acting resists. Negative-acting resists provide an image complementary to that of the photomask and characteristically have a high chemical resistance; good image reproduction qualities; and are of low cost. They have been and are widely used in the manufacture of microelectronic devices for these reasons. Because of their high chemical resistance, the negative-acting resists are generally more difficult to remove than other resists, although there are satisfactory commercial strippers.
Positive-acting resists are totally different from negative resists in response to actinic light and the resulting image, although the essential composition is similar. Both contain sensitizers, resins, solvents and additives. Positive resists provide an identical image, after exposure and developing, to that of the photomask and are colored and soluble in strongly alkaline solutions. They develop in mildly alkaline solutions. General chemical resistance is less than the negative resist, and positives are more costly to procure. However, images from these resists are extremely accurate, require minimal processing technique, and involve few processing steps. Moreover, the use of alkaline stripping and developing solutions greatly simplifies equipment selection by allowing low cost, readily available plastics to be used. However, positive resists suffer from disadvantages in that developing is expensive; it is time consuming; and it must be accurately controlled.
Processes using photographic emulsions having a light sensitive silver coating are also time consuming and difficult, as well as expensive and hard to control. At present there are three primary manufacturers of glass plates having micro images for semiconductor masks. The method of processing the glass plates is similar in all cases. The exposed plate can be processed for a like (positive) image or negative (normal or straight) image depending on chemical sequences. The negative image involves development in an alkaline developer, a wash, treatment in an acid bath to cause the unexposed silver to be water soluble, a final washing and a drying period.
Should a like or positive image be desired the exposed plate must be processed as follows: (1) alkaline development; (2) wash; (3) bleaching out of the developed silver in an acid bath; (4) wash; (5) an alkaline bath to neutralize the acid absorbed into the emulsion; (6) a wash and re-exposure of unexposed silver to light; (7) a re-development in an alkaline developer; (8) a wash; (9) treatment in an acid bath to harden the developed silver image; (10) a final wash; and (11) a drying period.
The many disadvantages apparent are chemistry cost, time (20 minutes for negative, 45 to 60 minutes for positive), contamination, necessity of critical control of all stages, expensive processing equipment costs, and resultant grain effects on image edges.